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Thorax:Venous structure:Azygous vein
The azygos vein is a unilateral vessel that ascends in the thorax to the right side of the vertebral column, carrying deoxygenated blood from the posterior chest and abdominal walls. It forms part of the azygos venous system and offers a collateral pathway between superior and inferior venae cavae in cases of venous obstruction Origin The azygos vein is formed by the union of the ascending lumbar veins and right subcostal veins at the level of T12. Course * It passes through the aortic opening of the diaphragm under the right crus, lateral to the thoracic duct * It passes upwards lying on right side of the anterior vertebral bodies of T12 to T5 vertebral bodies. * At level T4, the azygos vein arches forward over the hilum of the right lung and enters the it joins the SVC. This arch of the azygos vein (arcus venae azygos) is an important anatomical landmark. Tributaries * hemiazygos vein is a similar structure on the opposite side of the vertebral column and drains into the azygos vein at T7 and T8 vertebra. * right posterior intercostal veins: 5 to 12th posterior intercostal. * right superior intercostal vein: formed by 2 to 4th intercostal veins * right superior phrenic vein * tracheal veins * oesophageal veins * bronchial veins from right lung * pericardial veins The azygos vein freely anastomoses with the vertebral venous plexus. Variant anatomy * azygos fissure and azygos lobe * absence of the azygos vein (rare) * azygos continuation of the IVC The hemiazygos vein is the asymmetric counterpart to the azygos vein and forms part of the azygos venous system. The hemiazygos vein is formed by the confluence of the left ascending lumbar and left subcostal veins in the abdomen (and often communicates with the left renal vein). Course * The hemiazygos vein enters the thorax directly through the diaphragmatic crura. * It courses superiorly on the left side of anterior T12 to T9 vertebral bodies. * At level of T8 or T9 vertebral bodies, it crosses the midline anteriorly to the vertebral column to drain into the azygos vein. Tributaries * Left posterior 9th-12th intercostal veins (Five) * left superior phrenic vein * Esophageal and mediastinal veins * left renal vein (ocassionally) * IVC (ocassionally) The accessory (or superior) hemiazygos vein forms part of the azygos system and along with the hemiazygos vein it is partially analogous to the right-sided azygos vein. It drains the left superior hemithorax. The accessory hemiazygos vein is formed by the confluence of the middle left posterior intercostal veins. It descends on left side of anterior T4 to T8 vertebral bodies and crosses posteriorly to the aorta at the level of T7-8 to drain into the azygos vein. It normally anastomoses with the left superior intercostal vein. Tributaries * left 5th-8th posterior intercostal veins (four) * left bronchial veins (variable) Variant anatomy * drains via a common trunk with the hemiazygos vein into the azygos vein * forms a common trunk with the hemiazygos vein that passes anterior to the aorta called the interazygos vein * drains directly into the left brachocephalic vein (rare) The left superior intercostal vein drains the left posterosuperior hemithorax and is considered to be part of the azygos venous system even though it does not directly drain into the azygos vein. Origin Union of the 2nd to 4th left posterior intercostal veins. Course It courses superiorly to the left of the midline between the vagus and phrenic nerve, arches posteriorly lateral to the aortic arch to drain into the left brachiocephalic vein. It typically communicates with the accessory hemiazygos vein. Tributaries * First two or three left intercostal veins * Left bronchial veins * Left pericardiacophrenic vein Variant anatomy * may be enlarged in the presence of congenital azygos, hemiazygos or accessory hemiazygos vein absence (rare) * may not communicate with the accessory hemiazygos (25%) * left azygos (or hemiazygos) lobe may be caused by an abberrant left superior intercostal vein (analogous to the azygos lobe) * hemiazygos and accessory hemiazygos veins may drain directly into the left brachiocephalic vein via the left superior intercostal vein